Hi - the main thing to note is that you have sought medical advice because rectal bleeding is never normal. I still wonder whether your blood loss is substantial enough to cause anaemia which causes tiredness etc. Ask your doctor if it would be helpful to take iron tablets.
You are at the age when the more sinister diseases of the intestines can occur. Type in rectal bleeding to google/yahoo etc and click on one of the many sites to learn of possible causes/treatments etc.
A possible cause is diverticular bleeding. However, do insist on a proper diagnosis rather than a "well it might be .....". A colonoscopy is a comprehensive test that should reveal a lot more.
If the diagnosis shows that you are at risk from diverticular bleeding then it may be possible to identify the source with an angiogram and "cauterise" the site with the colonoscopy instrument.
I suffered torrential diverticular bleeding five years ago and, in order to eliminate the risk of re-occurrence while far away from a hospital (life threatening) I underwent a total colectomy (read about it on a previous posting):-
Hope this helps a little.